Ultrasonic sensors are believed to be understood in certain respects from the related art, which are used for distance measurement between vehicles and obstacles, for example, to assist a parking operation. The sensors have an oscillating diaphragm, which is generally excited via a piezo element in resonance to an oscillation. The sound signal generated in this way is emitted by the diaphragm of the ultrasonic sensor, reflected by an obstacle, and received by the same or an adjacent ultrasonic sensor. The distance between the sensor and the obstacle may be determined from the runtime of the sound signal. By applying the so-called trilateration principle, by way of comparison of the signals received by the emitting sensor itself (direct echo) and the signals received by adjacent sensors (cross echo), the position of the obstacle within the sensor plane may also be determined, in addition to the distance.
In general, the piezo element is situated on the base of a pot-like housing, which is installed, for example, in the bumper of the vehicle, the piezo element being connected to analysis electronics in such a way that the entirety of the unit forms the corresponding ultrasonic sensor. In order that the sound generated by the piezo element may be emitted or also received appropriately, the housings are shaped in such a way that they have a resonance in the frequency range of the ultrasonic waves used. The resonance is determined by the dimensions and the oscillation properties of the housing, in particular by the configuration of the diaphragm. Such ultrasonic sensors are discussed, for example, in DE 10 2008 040 905 A1.
Typically, at least four such sensors are installed on the rear end of the vehicle. For the reliable and precise determination of the distance to an obstacle, initially one of the sensors emits an ultrasonic signal. All sensors receive the echo signal. This operation is then carried out using each of the sensors in a fixed sequence until the final result of the measurement is provided and the cycle may begin from the beginning. The chronological resolution of the measurement is limited by the cycle duration.